The invention relates to catalysts useful for olefin polymerization. In particular, the invention relates to an improved method for preparing xe2x80x9csingle-sitexe2x80x9d catalysts based on heterocyclic ligands such as carbazolyl and quinolinoxy ligands.
While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry""s future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of xcex1-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Metallocenes commonly include one or more cyclopentadienyl groups, but many other ligands have been used. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. xe2x80x9cConstrained geometryxe2x80x9d or xe2x80x9copen architecturexe2x80x9d catalysts have been described (see, e.g., U.S. Pat. No. 5,624,878). Bridging ligands in these catalysts lock in a single, well-defined active site for olefin complexation and chain growth.
Other known single-site catalysts replace cyclopentadienyl groups with one or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S. Pat. No. 5,554,775 or azaborolinyl groups (U.S. Pat. No. 5,902,866).
U.S. Pat. No. 5,539,124 (hereinafter xe2x80x9cthe ""124 patentxe2x80x9d) and U.S. Pat. No. 5,637,660 teach the use of anionic, nitrogen-functional heterocyclic groups such as indolyl, carbazolyl, 2-pyridinoxy or 8-quinolinoxy as ligands for single-site catalysts. These ligands, which are produced by simple deprotonation of inexpensive and readily available precursors, are easily incorporated into a wide variety of transition metal complexes. When used with common activators such as alumoxanes, these catalysts polymerize olefins to give products with narrow molecular weight distributions that are characteristic of single-site catalysis.
One drawback of the catalysts described above is their relatively low activity. Normally, a large proportion of an alumoxane activator must be used to give even a low-activity catalyst system. For example, in the ""124 patent, Example 16, a bis(carbazolyl)zirconium complex is used in combination with methylalumoxane at an aluminum:zirconium mole ratio [Al:Zr] of 8890 to 1 to give a catalyst having a marginally satisfactory activity of 134 kg polymer produced per gram Zr per hour. In Example 22, a similar complex is used with less activator (i.e., [Al:Zr/h]=1956 to 1) to give a catalyst with an activity of only 10 kg/g Zr/h. The activator is expensive, and when it is used at such high levels, it represents a large proportion of the cost of the catalyst system. Ideally, much less activator would be needed to give a catalyst system with better activity.
Another drawback relates to polymer properties. While the ""124 patent teaches that catalysts made by its method give polymers with xe2x80x9ca narrow molecular weight distribution,xe2x80x9d the actual molecular weight distributions of polymers made with the bis(carbazolyl)zirconium dichloride catalysts of Examples 16 and 22 of this reference are not reported. In fact, the molecular weight distributions of these polymers would preferably be narrower. I found that the MWDs of polymers made using the ""124 catalysts are actually greater than 3 (see Comparative Examples 6-8 and 11-13, below).
In sum, there is a continuing need for single-site catalysts that can be prepared inexpensively and in short order from easy-to-handle starting materials and reagents. In particular, there is a need for catalysts that have good activities even at low activator levels. Ideally, the catalysts would produce, at low activator levels, polyolefins with desirable physical properties such as good comonomer incorporation, favorable melt-flow characteristics, and narrow molecular weight distributions.
The invention is a method for making single-site catalysts useful for olefin polymerization. The method comprises two steps. First, a nitrogen-functional heterocycle is deprotonated with an alkyllithium compound to produce an anionic ligand precursor. The heterocycle is an indole, carbazole, 8-quinolinol, 2-pyridinol, or a mixture thereof. In the second step, the anionic ligand precursor reacts with about 0.5 equivalents of a Group 4 transition metal tetrahalide (or with about 1 equivalent of an indenyl Group 4 transition metal trihalide) in a hydrocarbon solvent at a temperature greater than about 10xc2x0 C. to give a mixture that contains the desired organometallic complex.
Catalyst systems comprising the organometallic complex-containing mixtures and an activator, as well as olefin polymerization processes that use the catalyst systems, are also included.
The complex-containing mixture actively polymerizes olefins, even when used with an exceptionally low level of an activator. Solvent dilution further enhances catalyst activity. In addition, the resulting polymers have a favorable balance of physical properties, including narrow MWD. The method provides a simple route to a variety of heterocycle-based, single-site catalysts and reduces the overall cost of these systems by reducing the amount of costly activator needed for high activity.
Catalyst systems prepared by the method of the invention comprise an activator and an organometallic complex-containing mixture. The catalysts are xe2x80x9csingle sitexe2x80x9d in nature, i.e., they are distinct chemical species rather than mixtures of different species. They typically give polyolefins with characteristically narrow molecular weight distributions (Mw/Mn less than 3) and good, uniform comonomer incorporation.
The organometallic complex-containing mixture includes a complex that contains a Group 4 transition metal, M, i.e., titanium, zirconium, or hafnium. Preferred complexes include titanium or zirconium. The mixture also normally includes unreacted starting materials and lithium halides.
In one aspect, the invention is a method for preparing the organometallic complex-containing mixture. The method comprises two steps: deprotonation of the ligand, and reaction of the anionic ligand precursor with a Group 4 transition metal tetrahalide.
In the first step, a nitrogen-functional heterocycle is deprotonated with an alkyllithium compound. Suitable nitrogen-functional heterocycles are indoles, carbazoles, 8-quinolinols, and 2-pyridinols. These compounds can have substituents that do not interfere with deprotonation or the subsequent reaction with the transition metal halide. Many of these compounds are commercially available or are easily synthesized. For example, indole, carbazole, 8-quinolinol, and 2-pyridinol are all inexpensive and commercially available, and many indoles are easily made from arylhydrazones of aldehydes or ketones and a Lewis acid using the well-known Fischer indole synthesis (see J. March, Advanced Organic Chemistry, 2d ed. (1977), pp. 1054-1055, and references cited therein). Additional examples of suitable nitrogen-functional heterocycles are described in U.S. Pat. Nos. 5,637,660 and 5,539,124, the teachings of which are incorporated herein by reference.
An alkyllithium compound is used to deprotonate the nitrogen-functional heterocycle. Suitable alkyllithium compounds can be made by reacting lithium with an alkyl halide. More often, they are purchased as solutions in a hydrocarbon (e.g., toluene or hexanes) or ether (e.g., diethyl ether or tetrahydrofuran) solvent. Preferred alkyllithium compounds are C1-C8 alkyllithiums such as methyllithium, isopropyllithium, n-butyllithium, or t-butyllithium. n-Butyllithium is particularly preferred because it is readily available, relatively easy to handle, and effective.
Usually, equimolar amounts of the alkyllithium compound and the nitrogen-functional heterocycle are used to produce the anionic precursor. Deprotonation can be performed at any suitable temperature, preferably at or below room temperature. While the deprotonation reaction can be performed at temperatures as low as xe2x88x9278xc2x0 C. or below, it is preferred to perform this step at room temperature. Vigorous mixing is essential because the lithium salt of the anionic ligand tends to precipitate and forms a thick slurry. The reaction is usually complete within an hour or two. The resulting anionic ligand precursor includes a carbazolyl, indolyl, 8-quinolinoxy, or 2-pyridinoxy anion and a lithium cation.
In the second step, the anionic ligand precursor reacts with a Group 4 transition metal tetrahalide. Suitable Group 4 transition metal tetrahalides include zirconium, titanium, or hafnium, and four halide groups, which may the the same or different. Suitable tetrahalides include, for example, zirconium tetrachloride, dibromozirconium dichloride, titanium tetrabromide, zirconium tetraiodide, hafnium tetrachloride, and the like, and mixtures thereof. Zirconium tetrachloride and titanium tetrachloride are preferred.
Reaction of about 0.5 equivalents of the Group 4 transition metal tetrahalide with one equivalent of the anionic ligand precursor gives an organometallic complex-containing mixture that includes the desired bis(carbazolyl), bis(indolyl), bis(2-pyridinoxy) or bis(8-quinolinoxy) complex. The reaction is performed at temperature greater than about 10xc2x0 C., which is not only convenient, but gives the best results. Preferably, the reaction occurs at a temperature within the range of about 15xc2x0 C. to about 60xc2x0 C.; most preferably, the reaction is simply performed at room temperature. The reaction is usually complete within a few hours, but it is often convenient and desirable to allow the reaction to proceed overnight (about 16-18 hours) at room temperature.
The preparation of the organometallic complex-containing mixture is performed in the presence of a hydrocarbon solvent. Preferred hydrocarbons are aromatic, aliphatic, and cycloaliphatic hydrocarbons having from 4 to 30 carbons, preferably 4 to 12 carbons, because these are conveniently removed from the mixture. Examples include pentanes, hexanes, cyclohexane, octanes, toluene, xylenes, and the like, and mixtures thereof.
When the reaction is complete, the mixture is preferably just concentrated by solvent removal under a stream of nitrogen or with vacuum stripping to give a solid residue that contains the desired organometallic complex in addition to some unreacted starting materials and some lithium halide salt as a by-product. This mixture commonly contains as much as 50 wt. % of recovered starting material (e.g., carbazole). Nonetheless, this residue is well-suited for use xe2x80x9cas isxe2x80x9d in a subsequent olefin polymerization. Also suitable, although less desirable, is to filter a solution of the organometallic complex-containing mixture to remove insoluble by-products.
Preferred organometallic complexes have the general structure LLxe2x80x2MX2, wherein M is zirconium or titanium, X is a halogen, and each of L and Lxe2x80x2, which may be the same or different, is selected from the group consisting of indolyl, carbazolyl, 8-quinolinoxy, and 2-pyridinoxy. More preferably, X is Cl or Br.
In a second method of the invention, the anionic ligand precursor is instead reacted with about one equivalent of an indenyl Group 4 transition metal trihalide under the conditions described above. The indenyl Group 4 transition metal trihalide is conveniently made according to well-known methods by reacting an indenyl anion with a Group 4 transition metal tetrahalide. The indenyl anion is produced by deprotonating indene with a potent base such as an alkyllithium compound or a Grignard reagent. Examples 28-30 below illustrate the second method.
Organometallic complex-containing mixtures of the invention are normally combined with an activator when they are used to polymerize olefins. As illustrated below in Example 2, the activator is commonly mixed with the complex just prior to use as a catalyst.
Suitable activators are well known in the art. Examples include alumoxanes (methyl alumoxane (MAO or PMAO), modified methyl alumoxane (MMAO), ethyl alumoxane, diisobutyl alumoxane), alkylaluminum compounds (triethylaluminum, diethyl aluminum chloride, trimethylaluminum, triisobutyl aluminum), and the like. Suitable activators include acid salts that contain non-nucleophilic anions. These compounds generally consist of bulky ligands attached to boron or aluminum. Examples include lithium tetrakis(penta-fluorophenyl)borate, lithium tetrakis(pentafluorophenyl)aluminate, anilinium tetrakis(pentafluorophenyl)borate, and the like. Suitable activators also include organoboranes, which include boron and one or more alkyl, aryl, or aralkyl groups. Suitable activators include substituted and unsubstituted trialkyl and triarylboranes such as tris(pentafluorophenyl)borane, triphenylborane, tri-n-octylborane, and the like. These and other suitable boron-containing activators are described in U.S. Pat. Nos. 5,153,157, 5,198,401, and 5,241,025, the teachings of which are incorporated herein by reference. Alumoxanes are particularly preferred activators; methyl alumoxane is most preferred.
The amount of activator needed relative to the amount of organometallic complex depends on many factors, including the nature of the complex and activator, the desired reaction rate, the kind of polyolefin product, the reaction conditions, and other factors. Generally, however, when the activator is an alumoxane or an alkyl aluminum compound, the amount used will be within the range of about 0.01 to about 500 moles, preferably from about 0.1 to about 300 moles, of aluminum per mole of M. When MAO is used, it is preferably used at a [Al:M] molar ratio of less than about 500, more preferably less than about 300. When the activator is an organoborane or an ionic borate or aluminate, the amount used will be within the range of about 0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles, of activator per mole of M.
The ability to use low levels of an activator is a key advantage of the invention. As the examples below illustrate, MAO can be used at much lower levels than previously employed. While MAO is commonly used at [AI:M] molar ratios in the thousands (see U.S. Pat. No. 5,539,124 at Examples 16 and 22), I have now found that molar ratios as low as [Al:M]=200 or below can give catalysts with excellent activity when the complex is prepared as described herein. This is a valuable discovery because the activator is a major contributor to overall catalyst cost, and ways to reduce its use have long been sought by the industry.
Storage stability is another advantage of catalyst systems prepared by the method of the invention. As the results in Table 2 below confirm, aging has a dramatic negative effect on the activity of the catalysts made using the methods described in the ""124 patent. In contrast, catalysts made by the method of the invention retain excellent activity, and polymers made using the catalysts of the invention have consistently narrow MWDs, even after 75 hours of storage.
If desired, a catalyst support such as silica or alumina can be used. However, the use of a support is generally not necessary for practicing the process of the invention.
Catalysts made by the method of the invention are particularly valuable for polymerizing olefins. Preferred olefins are ethylene and C3-C20 xcex1-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and the like. Mixtures of olefins can be used. Ethylene and mixtures of ethylene with C3-C10 xcex1-olefins are especially preferred.
Many types of olefin polymerization processes can be used. Preferably, the process is practiced in the liquid phase, which can include slurry, solution, suspension, or bulk processes, or a combination of these. High-pressure fluid phase or gas phase techniques can also be used. The process of the invention is particularly valuable for solution and slurry processes.
The olefin polymerizations can be performed over a wide temperature range, such as about xe2x88x9230xc2x0 C. to about 280xc2x0 C. A more preferred range is from about 30xc2x0 C. to about 180xc2x0 C.; most preferred is the range from about 60xc2x0 C. to about 100xc2x0 C. Olefin partial pressures normally range from about 15 psig to about 50,000 psig. More preferred is the range from about 15 psig to about 1000 psig.
Catalyst concentrations used for the olefin polymerization depend on many factors. Preferably, however, the concentration ranges from about 0.01 micromoles per liter to about 100 micromoles per liter. Polymerization times depend on the type of process, the catalyst concentration, and other factors. Generally, polymerizations are complete within several seconds to several hours.
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.